1. Field of the Invention
This invention relates to a three dimensional interstitial catalyst support and to a three dimensional interstitial catalyst system comprising the support which is suitable for use in catalysing chemical (especially pressure-sensitive) reactions which require disturbed flow having components of motion transverse to the general flow of fluid through the support to be imparted to one or more liquids. The support and catalyst system are especially suitable for use in trickle feed three phase rections in which a liquid is trickle fed and a gas is fed into a reaction zone packed with an interstitial solid catalyst system. Trickle feed three phase reactions are used for example in the hydrogenation of crotonaldehyde or butyraldehyde. The invention also relates to a convenient method for making the support and to a process using the catalyst system comprising the support.
Efficient performance of a liquid/solid phase catalysed reaction requires disturbed flow of the fluid so as to ensure good contact between the liquid and the solid catalyst. In particular, in a trickle feed three phase reaction the liquid should thoroughly wet the solid otherwise only a low conversion of liquid to product occurs and also if two or more products are possible, then the selectivity towards the preferred product may be reduced.
2. Description of the Prior Art
To ensure good selectivity and conversion, the catalyst systems hitherto employed have comprised elements such as pellets, sintered powders, closely woven or closely knitted two dimensional wire screens or narrow bore tubes optionally stuffed with steel wool to increase the obstructiveness of the elements. Such elements pack closely so as to obstruct flows mechanically and so impart a transverse component to flows which would otherwise be mainly longitudinal of an elongated catalyst system. Despite close packing, such systems are interstitial in that interstices are defined by the adjacent packed pellets, or by the weave or knit of a screen or by the bore and packing arrangement of tubes. In practice the volume of the interstices is no more than 30% of the volume of the whole catalyst system so as to ensure adequate obstruction.
Elements used in such close packed catalyst systems comprised either unsupported catalyst component or catalyst component absorbed into supports such as zine oxide, alumina, magnesia, silica or silicates such as asbestos. However, with both types of element attrition seems to occur during use and the interstices become increasingly blocked by trapped particles so that increasing pressure gradients become necessary to maintain the reaction. In pressure-sensitive reactions, increased pressure often results in decreased selectivity and hence increased amounts of by-product. Eventually the reaction ceases to be worthwhile. For example in hydrogenation of crotonaldehyde or butyraldehyde, blockages and consequent necessary increases in pressure can limit the commercially useful life of a pelletised copper chromite catalyst system to as little as three days.
Pelletised catalyst systems present an additional general problem in that it is often necessary to shovel pellets manually around areaction zone in order to achieve the required pattern of interstices.
Closely packed catalyst systems present a specific problem when used in trickle feed three phase reactions. They are vulnerable to the interstices flooding (ie becoming filled exclusively with liquid). A reaction involving liquid and gaseous phases is impossible in flooded areas (neglecting any minor reaction which may be possible between liquid and dissolved gas). Another problem arising from flooded interstices is that flooded interstices canalise the gas into exclusively gaseous flows causing it to pass through the system without mixing and reacting with the liquid.
Interstitial catalyst systems having open metal supports have been proposed for use in the purification of exhaust gases coming from gasoline engines in motor vehicles. Typically such supports comprise a cylindrical so-called honeycomb structure containing spiral layers made by rolling up a pair of superimposed layers, one layer being relatively impermeable (for example a layer of stainless steel foil) and the other layer being a material which defines open interstices. As exhaust gases pass longitudinally through the support, the relatively impermeable layer inhibits radial flow so minimising eddying which would hinder escape of exhaust gases from the engine. Such catalyst systems are suitable for promoting gaseous reactions, but their suppression of radial flow makes them unsuitable for use in reactions involving liquids, especially trickle feed reactions where the imposition of transverse flow to liquids is essential.